The Longest Day

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June 17, 2002


The longest day of the year should be the perfect time to get a tan, right? But does all that exposure to the Sun also make us more vulnerable to the effects of solar storms?

The longest day, also called the summer solstice, falls on June 21 this year. The northern hemisphere is most tilted toward the Sun exactly at 6:24 a.m. PDT. The Sun rises and sets farther north on the horizon than at any other time of year, providing the northern hemisphere with the most hours of sunlight in a single day. Subsequently, the Sun will rise further south each day until the winter solstice in December. The word solstice is from Latin words meaning "the Sun" and "to stop," because early observers thought that the Sun appeared to stop dead, then turn around, in its seasonal movement north and south along the horizon at sunrise and sunset.

In the northern hemisphere, the Sun is higher in the sky throughout the day and its rays strike Earth at a more direct angle, causing the efficient warming we call summer. In the winter, just the opposite occurs: The Sun is at its southernmost point and is low in the sky. Its rays hit the northern hemisphere at an oblique angle, creating the feeble winter sunlight. Below the equator, the seasons are reversed.

Every once in a while, the Sun hiccups out a burst of magnetic energy from its almost invisible outer layer. These solar storms take just a couple of days to travel to Earth, where they can cause satellites to short out, confuse cell phone or pager systems and produce beautiful nighttime auroras in the high latitudes. Solar storms are not dangerous to humans on Earth - they can't even make a person more tan. However, astronauts in on the space shuttle or on the space station can be exposed to hazardous doses of radiation during intense events.

So on the day of the summer solstice, when our hemisphere is looking the Sun straight in the face, should we be more battered than usual by solar weather?

"The number of solar storms doesn't increase around the time of the June solstice, but over the past five or six years, the intensity of the storms does tend to be high at this time of year," said Dr. Walter Gonzalez, a visiting researcher at NASA's Jet Propulsion Laboratory and the head of the space science program at Brazil's National Institute for Space Research. "We're not really sure why this happens, so this is one of the things we are investigating."

"Earth also sees more solar storms near the days of the equinoxes, in March and September," said Dr. Bruce Tsurutani of JPL. "But there is nothing at the winter solstice, in December. That is a minimum."

So what is the solar weather going to be like on this year's longest day? "If NASA's Solar and Heliospheric Observatory detects a coronal mass ejection coming toward the Earth, we will have a two- to three-day warning of a possible solar storm," said Gonzalez. "When the solar gas reaches the Advance Composition Explorer spacecraft, positioned just upstream of the Earth, we will know the intensity of the impending storm one hour beforehand."

"If the Sun is active on that particular day and if Earth's magnetosphere gets disturbed, the longest day of the year might be very intense," Gonzalez said.

To keep up with the latest Sun activity, visit http://www.spaceweather.com.

Contacts: JPL/Martha Heil (818) 354-0850

Images

Magnetic bursts from Sun's surface.

Magnetic bursts from Sun's surface.

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NASA's Spacelab snapped this photo of an aurora at the South Pole.

NASA's Spacelab snapped this photo of an aurora at the South Pole.

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